O-Rings as piston rings

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tornitore45
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O-Rings as piston rings

Post by tornitore45 » Sun Jul 08, 2018 7:56 am

All people that tried this had positive results, so I am going for it in my new Hoglet.

Question is how much interference from the true OD of the O-ring to the Bore?
I have no feeling for the friction to be expected in this case.
The O-ring catalog dimension are 1.004 x 0.864 x 0.070
Should the piston groove be 0.864 and let the ring compress radially?
OR
Should the groove be less than 0.864 and let the ring compress crcumferentially?

Am I overthinking the subject?

My wild guess is 0.001 on a 1" bore which should be mirror smooth
Mauro Gaetano
in Austin TX

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GlennW
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Re: O-Rings as piston rings

Post by GlennW » Mon Jul 09, 2018 4:23 pm

According to my O-ring catalog:

Clearance between the piston and cylinder to be .005"

Groove diameter to be bore diameter minus .114"

Groove width to be .090"

This for a moving piston seal, but it does not specify if hydraulic or pneumatic.
Glenn

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tornitore45
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Re: O-Rings as piston rings

Post by tornitore45 » Mon Jul 09, 2018 7:39 pm

Thank you Glenn this give me something to work with

I suppose the catalog data is targeted to a slow moving application, like a faucet for example.
I feel with this dimensions the friction is too high for an IC engine application.

all in thousands
(140-114)/2=13 So the 70 section is compressed by 13 ~ 20% sounds like a lot
It appears like the ring is forced to reduce its circumference by 0.4% which is really a trifle, but is compressed from the inside.
This means that if the groove dimension, as given, result too shallow and yield too much friction, I can go back and deepen the groove.
Mauro Gaetano
in Austin TX

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warmstrong1955
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Re: O-Rings as piston rings

Post by warmstrong1955 » Mon Jul 09, 2018 8:30 pm

Go here, and download the Parker O-Ring manual: https://www.parker.com/literature/ORD%2 ... ndbook.pdf
I would have just attached it, but it was too large for the boards.
This is the bible of o-ring design.
Read the 'dynamic' design section, and then you can go to the charts for all the dimensions you will need.

I did both hydraulic and pneumatic seal design, and I am a fan of back-up rings, most especially in dynamic designs. They improve o-ring life greatly, and prevent nibbling, which is a common o-ring failure. I have widened many a groove, to add a back-up.

Bill
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GlennW
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Re: O-Rings as piston rings

Post by GlennW » Tue Jul 10, 2018 7:31 am

I doubt that you will find specific application data for what you are wanting to do, so most likely the "trial and error" method will come into play!

The info I posted is for a linear actuating cylinder with an O-ringed piston.

It seems to me the "real" Bill (seal killer) did a similar project and got it working using O-rings after some experimentation.
Glenn

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warmstrong1955
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Re: O-Rings as piston rings

Post by warmstrong1955 » Tue Jul 10, 2018 7:49 am

Not sure I recall correctly, but I think Bill's problem at first was seal drag. (friction) So Mauro's comment about 'deepening the groove', is a sound one.
So is improving the surface finish, improving lubrication, and using a higher durometer of o-ring, which will decrease the CoF.
I think I would just toward the 'loose' end (less squeeze) of the tolerance, since it is a low pressure application.

Other Bill
(not the Real Bill)
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GlennW
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Re: O-Rings as piston rings

Post by GlennW » Tue Jul 10, 2018 8:53 am

warmstrong1955 wrote:
Tue Jul 10, 2018 7:49 am
Not sure I recall correctly, but I think Bill's problem at first was seal drag. (friction) So Mauro's comment about 'deepening the groove', is a sound one.
That is what I was referring to with the "trial and error" method.
Glenn

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warmstrong1955
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Re: O-Rings as piston rings

Post by warmstrong1955 » Tue Jul 10, 2018 9:26 am

GlennW wrote:
Tue Jul 10, 2018 8:53 am
warmstrong1955 wrote:
Tue Jul 10, 2018 7:49 am
Not sure I recall correctly, but I think Bill's problem at first was seal drag. (friction) So Mauro's comment about 'deepening the groove', is a sound one.
That is what I was referring to with the "trial and error" method.
Yup! I was agreeing with you!
:)
Other Bill
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GlennW
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Re: O-Rings as piston rings

Post by GlennW » Tue Jul 10, 2018 10:36 am

I'm not used to that. :)
Glenn

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warmstrong1955
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Re: O-Rings as piston rings

Post by warmstrong1955 » Tue Jul 10, 2018 10:40 am

GlennW wrote:
Tue Jul 10, 2018 10:36 am
I'm not used to that. :)
No no.....Steve Graham is not used to that!
;)
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tornitore45
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Re: O-Rings as piston rings

Post by tornitore45 » Tue Jul 10, 2018 5:51 pm

Many modeler have found the solution but I did not pay attention to the numeric details when reading about specific projects.
Unless someone that has successfully run a model engine comes up with the number I will start with a shallow groove and work from there until the piston friction feels the same as an engine with cast iron rings.
Mauro Gaetano
in Austin TX

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Re: O-Rings as piston rings

Post by Harold_V » Wed Jul 11, 2018 3:17 am

tornitore45 wrote:
Tue Jul 10, 2018 5:51 pm
I will start with a shallow groove and work from there until the piston friction feels the same as an engine with cast iron rings.
I'm not sure that would work, as I suspect the coefficient of friction for an O ring is far greater than that of a well seated iron ring. Could be that by the time it felt "right" you'd be too loose and it would leak. Finish of the bore will be critical, especially in regards to ring life. A high polish would respond far differently from a rough honed surface, needless to say.

In any case, it's an interesting experiment. Be certain to post what you learn.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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